The present invention relates to a process for the preparation of organic polyisocyanates in the presence of solvents. More particularly, it relates to a process in which the solvent put into the process is reused after at least part of the solvent is treated with compounds containing isocyanate reactive hydrogen atoms.
Organic polyisocyanates are produced on an industrial scale by phosgenation of the corresponding primary polyamines in the presence of inert organic solvents such as chlorobenzene or ortho-dichlorobenzene (see, e.g., Ullmans Encyklopa die der technischen Chemie, 4th Edition (1977), Volume 13, page 350 et seq). In the production of polyisocyanates (particularly, hexamethylene diisocyanate, tolylene diisocyanates and polyisocyanates of the diphenyl methane series) by such a process traces of compounds containing isocyanate groups are invariably formed (e.g., 6-chlorohexyl isocyanate in the production of hexamethylene diisocyanate; tolyl isocyanate in the production of tolylene diisocyanates; and phenyl isocyanate in the production of polyisocyanates of the diphenyl methane series by the phosgenation of aniline/formaldehyde condensates). Such isocyanate group-containing compounds have an extremely deleterious effect on the quality of the end products. The amount of such deleterious isocyanates in the product polyisocyanates has been reduced by removing the deleterious isocyanates from the product polyisocyanate. The solvent which is distilled off with the unwanted isocyanates is subsequently treated in an elaborate column distillation to remove any impurities before the solvent is reused. This purification of the solvent by distillation is possible only with a considerable expenditure of energy and outlay in apparatus. Further, those undesirable compounds having a boiling point near the boiling point of the solvent used are extremely difficult or impossible to remove.